In many industrial processes it is desirable to contact dry air with chemicals or food produce to manufacture dried substances, e.g. in convective dryers. In order to produce dry air it is a common method to pass intake air from the surroundings through an adsorption dehumidifier thereby removing excess moisture from the intake air.
Adsorption dehumidifiers, e.g. desiccant dehumidifiers, remove water from a volume of air that passes through it. The heart of this process could be an adsorption rotor made from or coated with a special substance that absorbs the water molecules that make up the moisture in the passing air.
As the rotor absorbs water it becomes necessary to remove absorbed water in order to regain the adsorption capability of the rotor. In order to remove the humidity in the rotor, the rotor is rotated over to a regeneration zone, where it is dried with heated air. The warm, humid regeneration air is led out, and the rotor is once again ready to absorb water molecules. This operation can be continuous or stepwise.
In recent years an increased focus on process economics has led to improvements in dehumidifier designs focusing amongst other elements on the reuse of the energy content of the warm and humid regeneration air rather than merely discarding it to the atmosphere.
It is an aim of the present invention to suggest an improved system for regenerating a desiccant wheel wherein the regeneration air in the form of superheated steam is used for further purposes in a process stream associated with the desiccant wheel.
U.S. Pat. No. 3,183,649 describes a dehumidifier system comprising a rotary desiccant wheel which comprises a first means to supply a supply gas to an adsorption section of the desiccant wheel, second means to supply superheated steam to a regeneration section of the desiccant wheel, and a third means to supply a flush gas to a flush section.
EP 0014895 discloses an apparatus for producing liquid water from moisture in the air by adsorbing the moisture on an adsorbent and then desorbing water 5 from the adsorbent by heating it and condensing steam into liquid water. The water producing apparatus comprises a recycling passage for recycling steam through an adsorbent column in the desorbing step, a heater for heating the steam in the recycling passage; and a condenser branched from the recycling 10 passage. In the desorbing step, steam in the recycling passage is heated by the heater to heat the adsorbent and to desorb water from the adsorbent and excess of steam corresponding to the desorbed steam is passed from the recycling passage to the branched condenser and is condensed to obtain liquid water in high efficiency.
WO 2008/044932 describes a dehumidifier system comprising a zeolite rotary desiccant wheel which comprises a first means to supply a supply gas to an adsorption section of the desiccant wheel, a second means to supply superheated steam to a regeneration section of the desiccant wheel, and a third means to supply a flush gas to a flush section, whereby each of the first, second and third means comprises a ventilator or compressor. In an embodiment of the invention of the prior art the superheated steam circulates the zeolite rotary desiccant wheel in a closed-loop regeneration system thus creating an ever increasing surplus of steam with each regeneration cycle. In order to dispose of the surplus moisture the inventors of WO 2008/044932 suggest passing the excess of superheated steam through a heat exchanger to preheat the supply gas intended to be dried in the zeolite rotary desiccant wheel. Thereby, however, energy needed to maintain the temperature of the superheated steam is lost from the closed regeneration loop. FIG. 1 shows the dehumidifier of the prior art.
An advantage to the use of zeolite rotary desiccant wheels with superheated steam is given by the adsorption properties of zeolites at the high temperatures. However the regeneration of zeolite rotary desiccant wheels are less efficient unless steam temperatures in excess of 200° C. are used for regenerating the zeolite rotary desiccant wheel as the adsorption capacity of zeolites do not fall significantly below 5 wt % adsorbed water at lower temperatures. This increases energy consumption for heating the superheated steam to an adequate operating temperature, which can be difficult to maintain in an operating environment and heat loss from the regeneration flow path may be significant. Further such high operating temperatures also require heating of any supply gas entering the desiccant wheel in order not to cool the zeolite desiccant wheel below an efficient operating temperature for the intended steam recovery. Thereby a part of the energy advantage of using superheated steam for regeneration of a zeolite rotary desiccant wheel is lost.
It is an aspect of the present invention to improve the energy efficiency of the systems of the prior art by suggesting further uses for the excess of superheated steam generated in a closed-loop regeneration system based on superheated steam. It is further an aspect of the present invention to present an improved dehumidifier system for use with other rotary desiccant wheels besides zeolites, preferably rotary desiccant wheels for silica gels and/or activated alumina.
In one aspect, the present invention therefore relates to the regeneration of a silica gel rotary desiccant wheel using a closed-loop regeneration system based on superheated steam wherein excess superheated steam and energy generated during regeneration of the desiccant wheel is used in further energy requiring processes within the environment wherein the dehumidifier system of the present invention is installed. Thereby the superior absorption ability for water of silica gel or activated alumina desiccants used in rotary desiccant wheels can be combined with a closed-loop superheated steam recovery of such rotary desiccant wheels at lower operating temperatures and an increase energy gain from the regeneration process.